1. Technical Field
The present invention lies in the art of thermal mass transfer printing. More specifically the invention concerns a method and composition for an acceptor sheet for wax thermal transfer printing having improved wax receptivity for better resolution and a reduced tendency to jam the printing mechanism. In particular, the invention provides for the inclusion of a poly(alkylvinylether) in the coating on the acceptor sheet, in combination with another polymer with a higher glass transition temperature and a filler.
2. Description of the Prior Art
Thermal printing involves the selective application of heat to a heat sensitive material which results in the formation of images on the material. One category of thermal printing is the donor sheet-acceptor sheet system, whereby a thermal printhead applies heat to the backside of a donor sheet in selective imagewise fashion. The images are transferred to the acceptor sheet either by chemical reaction with, or mass transfer from, the donor sheet.
Chemical reaction systems provide for the volatilization of a chemical coating on the donor sheet at locations where the donor sheet is contacted by the thermal printhead. The volatilized chemical migrates from the donor sheet to the acceptor sheet where it reacts with a chemical coating to produce a visible image on the acceptor sheet. One example of this is the volatilization of a phenol compound on the donor sheet which reacts with a leuco compound on the acceptor sheet. The leuco compound is thus converted from a colorless to a colored form and produces a visible image.
Mass transfer systems provide for the transfer of colored material directly from the donor to the acceptor sheet, with no color-forming chemical reaction occurring.
In wax thermal (mass) transfer printing, an ink or other record-forming material in admixture with a wax compound is transferred from a donor such as a carrier ribbon to an acceptor sheet by applying heat to localized areas of the carrier. The wax/ink mixture on the carrier ribbon melts or softens, preferentially adhering to the acceptor sheet, which may be either paper or transparent film. In the case of paper, the acceptor sheet has more surface roughness than does the carrier, so ink transfer is largely achieved by a physical interlocking of the softened wax and ink with the paper fibers.
The transfer of ink to an acceptor sheet film such as transparent polyester, differs in that the surface of the film is very smooth. Here, wetting of the film surface by the softened wax/ink mixture must be adequate in order to provide preferential adhesion of the wax/ink mixture to the acceptor rather than to the donor sheet. The transfer of single pixel dots is particularly sensitive to differences in adhesion because some of the heat input at the individual dot is dissipated into the surrounding ink mass, decreasing the temperature of the dot and lessening its ability to transfer.
One solution to this problem has been to incorporate wax in a coating layer placed over an acceptor sheet film substrate.
U.S. Pat. No. 4,686,549 relates to a receptor (i.e., acceptor) sheet having a wax-compatible image receptive layer which can be inter alia an ethylene/vinyl acetate copolymer blended with a paraffin wax, a microcrystalline wax or a mixture of both. The image receptive layer has a critical surface tension higher than that of the donor sheet, which aids in wetting of the image receptive layer. Furthermore, this patent teaches that the Vicat softening temperature (as measured by ASTM D1525 (1982)) of the polymers forming the image receptive layer should be at least 30.degree. C. to prevent tackiness of the acceptor sheet at room temperature. At softening temperatures below 30.degree. C., according patent, problems arise such as fingerprinting and blocking of stacked film.
Polymeric coatings with a 30.degree. C. or higher softening point generally do have the advantage of minimal handling problems, as suggested by the above patent. The disadvantage is that such coatings are suitable for use only with selected combinations of printers and donor sheets. If, for example, the melting point of the wax on the donor sheet is above a specified maximum for a given printer, an insufficient amount of wax may be transferred to the acceptor sheet. Likewise, if the particular printer does not provide sufficient heat energy, the heat transfer from the donor sheet to the acceptor sheet, via the wax, may not increase the tackiness of the image receptive layer sufficiently for adhering the wax to the acceptor sheet, even if the wax does melt sufficiently for transfer. The result is inter alia poor fine line reproduction.
A number of polymeric coatings placed on the acceptor sheet have been claimed to improve ink transfer, including polyester, polycarbonate, polyamide, urea, and polyacrylonitrile resins, saturated polyester resins, stearamide, and poly(alkylvinylethers), poly(meth)acrylic esters, polymethylvinylketone, polyvinylacetate, and polyvinylbutyral.
In general, these polymeric coatings have a somewhat higher degree of adhesiveness than the transparent film substrate. This accounts for an increased receptivity of the coating as compared to the substrate. Heat transfer from the printing head to the coating increases adhesiveness even further.
Examples of this type of coating are disclosed in U.S. Pat. No. 4,678,687 which relates to thermal transfer printing sheets useful as transparencies wherein a polymeric coating is applied to a receptor substrate. The coating can be a poly(vinylether), poly(acrylic acid ester), poly(methacrylic acid ester), poly(vinylmethylketone), poly(vinylacetate) or poly(vinylbutyral). The coating allegedly provides increased resolution as compared to an uncoated substrate by increasing the adhesion of the transferred ink or dye to the receptor printing sheet. The coating composition is approximately 100% of the recited polymers.
A problem arises with these compositions when the tackiness of the coating is high enough to cause feeding problems and jamming of the printer due to adhesion either between acceptor sheets, or between the acceptor sheets and the printer rollers. High tackiness can also result in excessive wax transfer from the donor which, in the case of transfer of single pixels, results in unacceptable half tone images due to bridging of individual half tone dots. Excess tackiness also results in fingerprinting and blocking.
Hence, there remains a need in the art for an acceptor sheet which provides image formation, and particularly pixel dot image formation, of a quality sufficient for the printing of finely detailed images without loss of resolution in half tone images. There is also a need in the art for an acceptor sheet which can be used with a wider variety of printer/donor sheet combinations than has heretofore been possible and which simultaneously maintains adequate handling characteristics. These needs are met by the present invention.